1. Field of the Invention
This invention relates to the field of altitude measurement and, more particularly, to high precision altitude measurement for use by aircraft.
2. Description of the Related Art
For obvious reasons, it is important to accurately know, at all times, the altitude of an aircraft in flight. The importance is raised by an order of magnitude in the field of military aircraft, where the altitude may need to be measured and known to very strict tolerances under hostile conditions. Military aircraft often travel surreptitiously, and have special needs for highly accurate measurements of their current altitude above ground level (xe2x80x9cAGLxe2x80x9d).
Conventionally, when measuring AGL altitude, aircraft use radar altimeter readings which are detectable and, hence, carry significant risk in hostile territory. Radar altimeters operate by emitting a radar beam, either continuous or pulsed at close time intervals. The beam is directed from the in-flight aircraft toward underlying local terrain (i.e. toward terrain proximate the aircraft current position), and the duration of the period from broadcast of the beam to reception of the reflected return signal is used to determine the AGL altitude. However, enemy ground monitors may detect the radar altimeter beam and initiate anti-aircraft measures to intercept and destroy the aircraft. Thus, the use of constant or substantially constant radar emissions for determining current aircraft altitude must be avoided.
It is of course known to measure altitude above mean sea level (xe2x80x9cMSLxe2x80x9d) as a function of static air pressure detected on the exterior of the aircraft. This technique is commonly employed in commercial and general aviation as the primary means of determining altitude. However, this method does not measure AGL altitude, so the combat pilot is still left with an unknown error correction representing the height of the aircraft above the terrain, and/or other obstacles (e.g. buildings).
There is yet another problem with relying on static air pressure as a determinant of the AGL altitude of the aircraft. Air pressure varies not only as a function of the MSL altitude of the aircraft, but also with the local temperature and barometric pressure. When used in commercial or general aviation, local temperature may be easily measured and aircraft may correct for barometric variations by securing current barometric readings from local airport transmissions. Obviously, however, military aircraft operating in hostile or enemy territory cannot expect to obtain or depend on reliable barometric information from local airports in the hostile territory.
There is therefore a need in the art for a high accuracy system and method for determining aircraft altitude without use of a substantially continuous radar beam, or reliance upon local airports to provide current barometric pressure information.
It is therefore an object of the invention to provide a reliable and accurate system and method for measuring the altitude above ground level (AGL) of a military aircraft operating over hostile terrain.
It is a more specific object of the invention to provide a system and method for measuring the altitude above ground level of an operating over terrain without using a substantially continuous radar or other reflected beam.
Briefly stated, the invention is directed to a system and method for providing highly accurate measurements of the altitude above ground level of an in-flight aircraft. The invention preferably employs Global Positioning System (xe2x80x9cGPSxe2x80x9d) data and/or Inertial Reference System (xe2x80x9cIRSxe2x80x9d) data for determining the current locationxe2x80x94i.e. the latitude and longitude coordinatesxe2x80x94of the aircraft. With this two-dimensional data, and reference to a topographical map or database or other source of ground elevations of the terrain over which the aircraft is positioned or proximate to the flying aircraft, the MSL elevation of the currently-overflown local terrain can be determined. The AGL altitude of the aircraft is established by emitting toward the local terrain a single short-duration signal pulse from the aircraft and receiving a return signal reflected back from the terrain, and measuring the period of reflection as an indicator of aircraft AGL altitude of the aircraft above the terrain. The MSL altitude of the aircraft is then calculated from the sum of the MSL elevation of the terrain and the AGL altitude of the aircraft. The aircraft may then compare the calculated sum to its estimated MSL altitude, determined using its standard static air pressure measurements, to calculate a barometric correction factor that will render its conventional altimeter accurate. Specifically, a local barometric correction factor is derived from the difference of the calculated and estimated MSL altitude measurements and that correction factor is then used to monitor the MSL altitude of the aircraft as it continues flight over the local terrain without having to emit signal pulses until the aircraft flies to another, widely-separated locator over the terrain.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.